238 On the Evolution of the Vertebrata, { March, 
history, however, the tribe with most contracted fins, Actinopteri, 
appeared in the Coal measures (Paleoniscide), or very soon after 
the Crossopterygia in the Devonian. 
The descent of the fishes in general has witnessed, then, a 
contraction of the limbs to a very small compass and their sub- 
stitution by a system of accessory radii. This has been an ever 
widening divergence from the type of the higher Vertebrata, and 
from this standpoint, and also a view of the “ loss of parts without 
complementary addition of other parts,” may be regarded asa 
process of degradation. 
Taking up the great division of the Actinopteri, which em- 
braces most of the species of living fishes, we can trace the direc- 
tion of descent largely by reference to their systematic relations 
when we have no fossils to guide us. 
The three subtribes adopted by Jordan represent three series of 
the true fishes which indicate lines of descent. The Holostei 
include the remainder of the old ganoids after the subtraction of 
‘the Crossopterygia and the Chondrostei. They resemble these 
forms in the muscular bulbus arteriosus of the heart and in the 
chiasm of the Optic nerves. Both of these characters are com- 
plexities which the two other divisions do not possess, and which, 
as descendants coming later in time, must be regarded as inferior, 
and therefore to that extent degenerate. Of these divisions the 
Physostomi approach nearest the Holostei, and are indeed not 
distinctly definable without exceptions. The third division, or 
Physoclysti, shows a marked advance beyond the others in: (1) 
The obliteration of the primitive trachea, or ductus pneumaticus, 
which connects the swim-bladder and cesophagus; (2) the advance 
of the ventral fins from the abdomen forwards to the throat; (3) 
the separation of the parietal bones by the supraoccipital ; (4) the - 
presence of numerous spinous rays in the fins; and (5) the 
roughening of the edges of the scales, forming the ctenoid type. 
There are more or less numerous exceptions to all of these char- 
acters. The changes are all further divergences from the other 
vertebrate classes, or away from the general line of ascent of the 
vertebrate series taken as a whole. The end gained is specializa- 
_ tion, but whether the series can be called either distinctively pro- 
_ gressive or retrogressive is not so clear. The development of 
= osseous spines, rough scales and other weapons of defense, to- 
a gether with the generally superior energy and tone which prevail 
